Reference throughout this specification shall be made to use of the present invention with respect to electric fence systems, however it should be appreciated that the principles of the present invention can be applied to other systems as well.
Electric fence systems, whether powered by electric fence energisers for stock control or forming part of security systems have a number of unique features and associated problems.
Electric fence systems are becoming more and more sophisticated and in recent times there has been a number of developments in which electric fence lines are used to carry communication signals. These signals are generally used for turning electric fence energisers on or off and/or checking the status of various sectors of the electric fence system. Some of these developments are described in New Zealand Patent Application Nos. 245206 and 239506.
Unfortunately, there are a number of problems associated with electric fence energisers which makes communications along the electric fence line difficult to achieve successfully.
One major problem associated with electric fence systems is that they comprise of literally kilometres of wire. The fence wires tend to act like a high gain aerial and readily pick up any electrical signals or noise in the region. This noise can include radio waves, pulses from electric fence energisers on neighbouring farms, static electricity build-up from the atmosphere, magnetic fields from nearby mains transformers, radio frequency interference from vehicles and other sources. All this electrical noise can mean that communication signals sent along the fence line are either not detected by the appropriate receivers (as the communications signal is masked by the noise), or that the receivers interpret incorrectly the noise as being a communication signal. Either way, the presence of electrical noise along the fence line can lead to incorrect operation of the electric fence system.
A preferred communications method described in the previous patent applications is that of pulse position modulation (PPM). Pulse position modulation unlike other types of communication signals, such as those which use amplitude modulation does not suffer from attenuation as the signal propagates along the electric fence line. Attenuation does not affect pulse position modulation because as the signal goes down the line, the time between each pulse does not change, even though the amplitude of the pulse may diminish. Unfortunately some of the noise present on an electric fence line is high frequency noise. This increases the possibility of signals being picked out from the noise which correspond to the time difference expected in a communications signal under a pulse position modulated system.
A further problem associated with electric fence systems is that the amount of noise on an electric fence line is not constant and varies from electric fence system to electric fence system and can also vary for a single electric fence system over a period of time. This variation can occur as a result of many factors such as the load on the electric fence line which can change depending on whether there are dry or wet conditions, whether there is grass or other objects touching the electric fence line and whether there are poor connections in the electrical fence system such as those resulting from improperly connected fence wires. Other factors contributing to the varying electrical noise include the possible presence of electrical noise sources such as radio stations, transmission wires, neighbouring farms, heavy electrical machinery such as pumps and the like.
Because of this varying electrical noise and the associated unpredictability, it is very difficult to design a communications system for an electric fence line that can, no matter what the conditions, guarantee to filter out the appropriate amount of electrical noise while still allowing a sufficiently strong communications signal to be received by receivers in the electric fence system.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.